This invention relates in general to capacitor build-out circuits (BOC) and in particular, to a circuit for producing a variable capacitance from a fixed capacitor utilized in an active network.
The electrical loss of cables increases with higher frequency and with longer distance. Along with these higher losses is associated the increasing difficulty of hearing and being heard while conversing with a second party on the line. Therefore, at some distance from the central office, the cable pairs must be treated with other devices in order that voice recognition and naturalness of speech is not lost. Inductive loading is one approach to reducing these cable losses.
An infinite number of different loading schemes are possible by using different values of inductance, different intervals between load points, and different cables.
Loading systems which have become standardized are designated by a letter which signifies the nominal distance between adjacent loading coils and a number which identifies the amount of inductance, in millihenrys, per loading coil. The three most prevalent spacing intervals are 3000, 4500, and 6000 feet used with a 44, 66 and 88 millihenry coil and designated B, D and H respectively.
The secondary parameters of cable pairs are substantially improved by loading. As a result, loading is used extensively in non-toll office-to-office trunks, toll connecting trunks and subscriber loops. The changes in secondary parameters due to loading are:
1. The attenuation is reduced within a certain pass band region. Beyond this region the attenuation becomes infinite. The frequency at which the attenuation approaches infinity is defined as the cutoff frequency.
2. The impedance within the pass band region is higher than that for the same pair without loading. It increases gradually with increasing frequency and as the cutoff frequency is approached it increases rapidly.
3. The velocity of propagation is substantially reduced.
4. In the voice frequency range, the impedance of loaded cable pairs are higher and more uniform then those of non-loaded pairs.
In a two wire voice frequency repeater a precise impedance must be realized to balance the line impedance in the Wheatstone bridge type circuit called the telephone hybrid. The balance network could be relatively simple if the two wire part is terminated with the H-88 loaded line having the near end section equal to 6,000 feet or D-66 loaded line having a 4500 feet end section. In practical situations the H-88 end section could vary from 0 to 6,000 feet (0 to 4500 feet for D-66). In such an application, the capacitive build out circuit (BOC) provides an adjustable shunt capacitance so that the effective end section length of the loaded pair may be built out to the nominal section length.
The addition of a capacitance build-out circuit must not disturb the longitudinal balance of the line, i.e., the impedance between tip and ground must be the same as the impedance between ring and ground. Standard practice is to implement build out capacitors using a large number of capacitors in binary progression and switching them in and out in various combinations to achieve the desired value. For N capacitor switch combinations, 2.sup.N build out capacitor values could be implemented. Such a method of implementing build out capacitance is expensive, bulky and space consuming. It also is not practical for logic control or other forms of electronic control.
The present invention provides a novel solution to this problem not found in the prior art.